1. Field of the Invention
The present invention relates to a method for designing a magnetron for microwave ovens, and more particularly, to a method for designing a magnetron for the microwave ovens, which is capable of improving in suppressing harmonics in over a fourth higher harmonic, especially by employing an improved choke put inside an anode seal which is an outputting block of the magnetron.
2. Discussion of Related Art
A conventional magnetron for microwave ovens, as shown in FIG. 1, is composed of a cathode 1 of a filament type set on a central position therein, a cylindrical metal anode 2 installed around the cathode 1, a vane 7 fixed onto the inner side of the anode 2, for providing a plurality of resonant cavities radially directed toward the cathode 1, an antenna feeder 8 electrically partial-connected to the vane 7, for transmitting energy of an electron group, an upper magnet 10 fixed to an upper yoke 5 and a lower magnet 10' fixed to a lower yoke 6, which forms a magnetic closed circuit for applying a magnetic field into an operational space 9 between the cathode 1 and the anode 2, an upper magnetic pole 11 and a lower magnetic pole 11' which provide paths of a magnetic circuit, an anode seal 3 serving as a supporter of the magnetic circuit path and body, an outputting ring 16 and a choke 17 for suppressing a fifth higher harmonic generated through the anode seal 3, and a discharge tube 19 for suppressing second and third higher harmonics.
The reference label 4 in FIG. 1 indicates a filament seal, reference label 12 is a cooling fin, reference label 13 is a filter box for eliminating nonessential radiation generation through an applying, reference label 14 is a high-tension condenser, reference label 15 is a choke coil for preventing components of the higher harmonic from counterflowing into the power supply and reference label 18 is an outputting cap.
Describing operations of the conventional magnetron for the microwave ovens with such a construction referring to FIGS. 1 to 3, first, the magnetic field of the permanent magnets 10 and 10' forms a magnetic circuit through the upper and lower yokes 5 and 6 and the upper and lower magnetic poles 11 and 11', to thereby form the magnetic field in the operational space between the cathode 1 and the anode 2 and apply magnetism thereinto.
By providing an electric field into the operational space 9 by the way of a supply of the power current to the cathode 1, there is radiated thermoelectron from the cathode by a mutual operation between the electric and magnetic fields.
The radiated thermoelectron is converted into high frequency energy, which is energy of an electron group, doing a cycloid movement in the operational space 9 by a plurality of the resonant cavity formed by the vane 7. The high frequency energy is transmitted to the vane 7 and output to inside a cavity of the microwave oven through the antenna feeder 8 connected to the vane 7.
At this time, 2450 MHz high frequency, for example, is generated in the resonant cavity by the anode 2 and the vane 7, and beyond this wave, at the same time the component of high harmonics having frequency corresponding to positive integer times is generated therein.
Such harmonic component is output into the cavity of the microwave oven together with the basic wave, then it is to easy in a leakage of electromagnetic wave from the microwave oven outside enough to be shorten in a wavelength of the harmonic, due to difficulty of the electromagnetic shielding.
Despite very weak leaked harmonic, such leakage harmonic is not only harmful to the human body but also causes an obstacle. To limit a radiation or a leakage of the non-necessary harmonic components, there is a filter on which a coil and a condenser are combined on its inputting side and a choke 17 made of a cylindrical metal body is combined inside the anode seal 3 of its outputting side as shown in FIG. 1.
The choke 17 is brazed on the anode seal 3 in such way that it surrounds the antenna feeder 8 inside the anode seal 3, to suppress the harmonic, height of the choke 17 including a concave part having a length of .lambda./4 of the harmonic wavelength to be eliminated.
The outputting ring 16 set on the outside of the anode seal 3 supports a gasket base, eliminating the harmonic.
In designing a choke for suppressing high harmonic inside such anode seal 3, a parallel resonance circuit system is used, that is, in the system the power current of specific frequency does not flow in the neighborhood of the choke by providing the choke with infinite impedance related to the restricted specific frequency, without considering a size of choke peripheral parts.
Explaining in detail the magnetron made on the basis of the parallel resonance circuit system of such harmonic suppressing choke, reference is made to FIG. 2 showing a sectional view for a harmonic suppressing choke of a magnetron. FIG. 2 is a schematic view of the higher harmonic suppressing choke 17 on which an opening part as a slot L of a loop type is directed upwards.
In FIG. 2, .o slashed..sub.1 ' provides an outer diameter of an antenna and .o slashed..sub.2 ' shows an inner diameter of an choke opening part. .o slashed..sub.3 ' indicates an outer diameter of the choke opening part and .o slashed..sub.4 ' is an appears an inner diameter of an anode seal formed as a slot L of a loop type, and La' designates a length of a choke.
FIG. 3a is a flow view of power current for the choke based on the parallel resonance circuit system and FIG. 3b depicts FIG. 3a as an equivalent circuit. In this equivalent circuit, an impedance Z and a specific impedance Z.sub.0 may be shown as the following numerical expressions. EQU Z=iZ.sub.0 tan(2.pi.La'/.lambda.) [Numerical Expression 1] ##EQU2##
Meanwhile, in the parallel resonance circuit system, in considering a short-circuit face at an opening end of the slot L as the loop type forming the choke of FIG. 3a, its impedance Z is .infin., that is, in the numerical expression 1, there is the following relationship: La'=.lambda./4'.
Further, as understood in FIG. 3a, when Z is .infin., electric current I becomes 0, thus a transmission of the suppressed harmonic outside is shielded.
However, actually, the electric current I does not become 0 even though La' is .lambda./4 due to a flanging capacitor etc. among the choke and choke peripheral parts.
Accordingly, in the parallel resonance circuit system, the .lambda./4 length of suppressing harmonic is applied basically in a design of the choke, and respective parts of the choke are designed by selecting the index most prominent in the attenuation through an experiment in order to consider the impact of changing the value of flanging capacitor by the choke and its peripheral parts, e.g., an inner diameter of an anode seal .o slashed..sub.4 ', an outer diameter of an antenna .o slashed..sub.1 ', an inner diameter of the choke .o slashed..sub.2 ' and an outer diameter of the choke .o slashed..sub.3 '.
In a high harmonic choke of a conventional parallel resonance circuit system, however, an available permission range in the choke design is limited since a size of choke peripheral parts is not considered, furthermore its design is not precise. Thus, not only a harmonic suppression specific falls, but also an inconvenience in measuring the continuous attenuation specific and getting the optimum value is caused with much time consumption.